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تسجيل مستخدم جديدMonitoring Supergiant Fast X-ray Transients with Swift. Rise to the outburst in IGR J16479-4514
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P. Romano
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IGR J16479-4514 is a Supergiant Fast X-ray Transient (SFXT), a new class of High Mass X-ray Binaries, whose number is rapidly growing thanks to the observations of the Galactic plane performed with the INTEGRAL satellite. IGR J16479-4514 has been regularly monitored with Swift/XRT since November 2007, to study the quiescent emission, the outburst properties and their recurrence. A new bright outburst, reaching fluxes above 10$^{-9}$ erg cm$^{-2}$ s$^{-1}$, was caught by the Swift/BAT. Swift immediately re-pointed at the target with the narrow-field instruments so that, for the first time, an outburst from a SFXT where a periodicity in the outburst recurrence is unknown could be observed simultaneously in the 0.2--150 keV energy band. The X-ray emission is highly variable and spans almost four orders of magnitude in count rate during the Swift/XRT observations covering a few days before and after the bright peak. The X-ray spectrum in outburst is hard and highly absorbed. The power-law fit resulted in a photon index of 0.98$pm{0.07}$, and in an absorbing column density of $sim5times10^{22}$ cm$^{-2}$. These observations demonstrate that in this source (similarly to what was observed during the 2007 outburst from the periodic SFXT IGR J11215-5952), the accretion phase lasts much longer than a few hours.
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For the first time, Swift is giving us the opportunity to study supergiant fast X-ray transients (SFXTs) throughout all phases of their life: outbursts, intermediate level, and quiescence. We present our intense monitoring of four SFXTs, observed 2-3
times per week since October 2007. We find that, unexpectedly, SFXTs spend most of their time in an intermediate level of accretion ($L_{X}sim 10^{33-34} $ erg s$^{-1}$), characterized by rich flaring activity. We present an overview of our investigation on SFXTs with Swift, the key results of our Project. We highlight the unique contribution Swift is giving to this field, both in terms of outburst observations and through a systematic monitoring.
We present two years of intense Swift monitoring of three SFXTs, IGR J16479-4514, XTE J1739-302, and IGR J17544-2619 (since October 2007). Out-of-outburst intensity-based X-ray (0.3-10keV) spectroscopy yields absorbed power laws with by hard photon i
ndices (G~1-2). Their outburst broad-band (0.3-150 keV) spectra can be fit well with models typically used to describe the X-ray emission from accreting NSs in HMXBs. We assess how long each source spends in each state using a systematic monitoring with a sensitive instrument. These sources spend 3-5% of the total in bright outbursts. The most probable flux is 1-2E-11 erg cm^{-2} s^{-1} (2-10 keV, unabsorbed), corresponding to luminosities in the order of a few 10^{33} to 10^{34} erg s^{-1} (two orders of magnitude lower than the bright outbursts). The duty-cycle of inactivity is 19, 39, 55%, for IGR J16479-4514, XTE J1739-302, and IGR J17544-2619, respectively. We present a complete list of BAT on-board detections further confirming the continued activity of these sources. This demonstrates that true quiescence is a rare state, and that these transients accrete matter throughout their life at different rates. X-ray variability is observed at all timescales and intensities we can probe. Superimposed on the day-to-day variability is intra-day flaring which involves variations up to one order of magnitude that can occur down to timescales as short as ~1ks, and whichcan be explained by the accretion of single clumps composing the donor wind with masses M_cl~0.3-2x10^{19} g. (Abridged)
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ypical Be/X-ray binaries) with a peak luminosity of a few 1E36 erg/s. The quiescent level, measured only in a few sources, is around 1E32 erg/s. We are performing a monitoring campaign with Swift of four SFXTs (IGRJ16479-4514, XTEJ1739-302, IGRJ17544-2619 and AXJ1841.0-0536/IGRJ18410-0535). We report on the first four months of Swift observations, started on 2007 October 26. We detect a low level X-ray activity in all four SFXTs which demonstrates that these transient sources accrete matter even outside their outbursts. This fainter X-ray activity is composed of many flares with a large flux variability, on timescales of thousands of seconds. The lightcurve variability is also evident on larger timescales of days, weeks and months, with a dynamic range of more than one order of magnitude in all four SFXTs. The X-ray spectra are typically hard, with an average 2-10 keV luminosity during this monitoring of about 1E33-1E34 erg/s. We detected pulsations from the pulsar AXJ1841.0-0536, with a period of 4.7008+/-0.0004 s. This monitoring demonstrates that these transients spend most of the time accreting matter, although at a much lower level (~100-1000 times lower than during the bright outbusts), and that the true quiescence, characterized by a soft spectrum and a luminosity of a few 1E32 erg/s, observed in the past only in a couple of members of this class, is probably a very rare state.
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first year of intense Swift monitoring of four SFXTs, IGR J16479-4514, XTE J1739-302, IGR J17544-2619 and AX J1841.0-0536. We obtain the first assessment of how long each source spends in each state using a systematic monitoring with a sensitive instrument. The duty-cycle of inactivity is 17, 28, 39, 55% (5% uncertainty), for IGR J16479-4514, AX J1841.0-0536, XTE J1739-302, and IGR J17544-2619, respectively, so that true quiescence is a rare state. This demonstrates that these transients accrete matter throughout their life at different rates. AX J1841.0-0536 is the only source which has not undergone a bright outburst during our campaign. Although individual sources behave somewhat differently, common X-ray characteristics of this class are emerging such as outburst lengths well in excess of hours, with a multiple peaked structure. A high dynamic range (including bright outbursts) of 4 orders of magnitude has been observed. We performed out-of-outburst intensity-based spectroscopy. Spectral fits with an absorbed blackbody always result in blackbody radii of a few hundred meters, consistent with being emitted from a small portion of the neutron star surface, very likely the neutron star polar caps. We also present the UVOT data of these sources. (Abridged)
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